FLUID AND SILICATE GLASS INCLUSIONS IN ULTRAMAFIC AND MAFIC XENOLITHS FROM HIERRO, CANARY-ISLANDS - IMPLICATIONS FOR MANTLE METASOMATISM

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Cr-diopside- and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic- and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO2. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel + clinopyroxene +/- glass +/- other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass +/- CO2 +/- sulphide +/- silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110-degrees-C for the early ultramafic- and mafic-glass inclusions, and a maximum of 1260-1280-degrees-C for late inclusions of host basaltic glass. In most samples the CO2 inclusions show a wide range in homogenization temperatures (-40 to + 31-degrees-C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about -40-degrees-C, recorded in some of the smallest CO2 inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside- and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6-12 km.